1. Field of the Invention
The invention relates to combustible gas sensors, and more particularly to a combustible gas sensor with improved resistance to catalyst poisons. The invention also relates to a method for warning if a sensor has been exposed to a catalyst poison.
2. Description of Related Art
Catalytic bead combustible gas sensors have been widely used in industry to detect the presence of combustible gases and vapors for safety purposes and to provide a warning of potentially hazardous conditions before these gases and vapors reach explosive levels. Commercial catalytic bead sensors detect gases through the use of electrically heated helical filaments typically embedded within an oxide material such as alumina, silica, or thoria. A complete gas sensor is composed of two elements: a gas-sensing element and a compensating element, which are typically arranged in a Wheatstone bridge circuit. The gas-sensing element is formed by dispersing a precious metal catalyst such as palladium or platinum on the surface of the metal oxide to catalyze the combustion of the combustible gases. A compensating element is made so that combustible gases do not burn on its surface, but is placed in the circuit for the purpose of compensating for environmental effects such as humidity and ambient temperature, which affect both the gas-sensing and compensating elements. Such a combustible gas sensor is claimed and described, for example, in U.S. Pat. Nos. 3,200,011, 3,092,799, 4,313,907 and 4,416,911, and in Mosley, P. T. and Tofield, B. C., Solid State Gas Sensors, Adams Hilger Press, Bristol, England (1987).
A fundamental problem that arises in using this technology is that the precious metal catalyst is susceptible to poisoning or inhibition by certain compounds commonly present in workplace atmospheres. Examples of such compounds include organosilicons (e.g. hexamethyldisiloxane, decamethylcyclopentasilane), organoleads (e.g. tetraethyl lead), organophosphates (e.g. tributyl phosphate), sulfur-containing compounds (e.g. hydrogen sulfide), and halogenated hydrocarbons (e.g. carbon tetrachloride, trichloroethylene). Considerable research has been conducted in an attempt to alleviate the effects of catalyst poisons or inhibitors. Inert porous materials have been used to filter out the poisoning or inhibiting materials that have a relatively large molecular size. These filter materials are either incorporated into the gas-sensing element or applied as an external filter located in the path of gas diffusion.
U.S. Pat. Nos. 4,111,658 and 4,246,228 disclose a gas sensing element which is formed from a catalyst-loaded zeolite or a uniform mixture of oxidation catalyst particles and zeolite particles. The small pore diameters of zeolites (3 to 9 angstroms) allow catalyst poisons or inhibitors to diffuse relatively slowly into the inner part of the gas-sensing element compared to low molecular weight combustible gases such as methane, and thus the poisons are adsorbed and trapped by the zeolite particles to avoid rapid catalyst poisoning or inhibiting. U.S. Pat. No. 4,123,225 describes a sensing element that is provided with an outer layer of a non-catalytic porous material, which tends to prevent non-volatile poisoning residues from reaching catalytically active regions of the gas-sensing element. U.S. Pat. No. 4,560,585 also discusses the preparation of a sensing element using non-catalytic porous aluminum oxide and supported catalyst to build separate and alternating layers to obtain a poison resistant combustible gas sensor. The problem arising from this approach is that typical non-catalytic materials have only weak acid, base, or redox sites and thus do not trap poisons or inhibitors effectively. A thick layer of a non-catalytic material is necessary to filter out poisoning materials effectively but this configuration results in an increase in power consumption and a decrease in sensitivity and thus stability.
European Patent Application No. EP094863 and PCT published application WO 00/43765 disclose gas-sensing elements which are surrounded by a powdered zeolite or porous insulating materials such as silica and alumina. This method is very close to an external filter approach, where a gas sample passes through a separate filter that contains suitable materials such as active charcoal before it reaches the gas-sensing element. However, the gas-sensing element with an external filter has difficulty in detecting alcohols, ketones and combustible gases with a high molecular weight, such as hydrocarbons above heptanes, since the filter also blocks these combustible gases.
When a combustible gas sensor becomes poisoned, it loses sensitivity to the combustible gas or gases that it was designed to measure. Usually, this poisoned condition is not noticeable until the sensor is tested with test gases. Because combustible gas sensors are often used in ambient air environments where there can be no control over the types of poisoning materials that might be encountered, users are required to manually check sensor operation on a regular schedule. Since a sensor can be poisoned quickly when exposed to a poison-containing environment, frequent checking is needed to ensure the integrity of a detection system.
A number of patents have addressed this problem. For example, U.S. Pat. No. 3,960,495 discloses a method of continuously supplying a controlled small amount of a combustible gas or vapor to the vicinity of a gas sensor; inoperativeness of the gas sensor due to poisoning or malfunctioning is observed when the sensor ceases to indicate the presence of at least the controlled small amount of combustible gas or vapor. PCT published application WO 99/17110 discloses an assembly for verifying the response of a combustible gas sensor through the use of a hydrogen generator in an explosion proof housing of a combustible gas sensor. However, this method requires a hydrogen generator to be embedded in the sensor housing, which is not suitable for portable gas detection instruments due to size constraints. Furthermore, hydrogen and organic vapors are not ideal combustible gases to check whether a sensor has been poisoned since they are among the most easily combustible gases, and therefore their responses to the sensor are least affected by poisons.